Diode lasers, either as discreet diodes, diode arrays, or diode bars, have been utilized to pump a variety of host crystals. Early work by Baer et al. determined that the efficiencies of end-pumped lasers are greatly enhanced by a design that focusses the diode pump light to a diameter in the crystal that is smaller than the TEMOO mode diameter. See for example, U.S. Pat. Nos. 4,653,056 and 4,656,635. This classic discovery has been called "mode-matching".
The effect of mode-matching is to maximize the coupling between the TEMOO mode of the laser resonator and the excited volume in the crystal within the resonator. In turn, the optical slope efficiency and the overall optical efficiency are both maximized. In a classic mode-matched geometry, the ratio R of the TEMOO mode diameter to the pump beam diameter in a diode-end-pumped Nd:YAG laser is typically about 1.3 or greater. At lower pump powers, such as 2 W, mode-matching has proven to be very successful in achieving the required efficiencies and in producing gaussian beams that are very nearly diffraction limited.
At low pump powers, mode-matching techniques work well both for materials with strong thermal lens characteristics (like Nd:YAG and Nd:YV04), and for materials with weak thermal characteristics (like Nd:YLF). This is because pump powers below about 2 W are typically too low to induce a thermal lens of a magnitude that could significantly alter the properties of a typical diode-pumped laser resonator. At higher pump powers, such as those available from 10 and 20 W diode laser bars, thermal effects become significant in end-pumped lasers.
At the higher pump powers of diode bars, thermal lenses of appreciable focussing power can be generated in the laser crystal by the diode pump light. The aberrations that are inherently associated with these strong thermal lenses are thought to limit the efficiency of high power diode-pumped lasers. See for example S. C. Tidwell, J. F. Seamans, M. S. Bowers, A. K. Cousins, IEEE J. Quantum Electron. 24, 997 (1992).
"Strong" and "weak" thermal lensing as follows. With a "strong thermal lens", the focussing power of the pump-induced thermal lens is at least comparable to that of the other optics in the laser resonator. A strong thermal lens significantly changes the size and divergence of a laser resonator eigenmode within the laser resonator. With a "weak thermal lens", the focussing power of the pump induced lens is substantially lower than that of the other optics in the laser resonator, such as mirrors and typical lenses. The thermal lens can be considered weak if the other optics in the laser resonator dictate the size and divergence of the resonator eigenmode, while the thermal lens has little effect on the eigenmode properties.
High aberration materials have some material properties that make them suitable for certain applications. With respect to Nd:YLF, Nd:YVO4 exhibits high gain and a short upper state lifetime. Nd:YAG has an intermediate gain and an intermediate upper state lifetime. These properties provide important adjustable parameters when designing a Q-switched laser with high pulse energy or high repetition rate, or a laser that is insensitive to optical feedback. Additionally, certain properties of Nd:YVO4 make it attractive for diode pumping; its absorption coefficient at the diode pump wavelength of .about.809 nm is extremely high, permitting efficient coupling of diode pump light into the Nd:YVO4 crystal.
There are ready examples of materials with strong and weak thermal lens characteristics. Examples of materials that tend to exhibit strong thermal lenses are Nd:YAG and Nd:YV04. Pump-induced surface distortion can contribute to the thermal lens magnitude, but the effect is primarily due to a strong dependence of the material's index of refraction upon the local temperature in the material. While this dependence is about one order of magnitude smaller for Nd:YLF that it is for Nd:YAG and Nd:YV04, it should be noted that even the focussing, or defocussing, power of a thermal lens in a material like Nd:YLF must be considered in a laser resonator if its focussing power is comparable to that of the other intracavity optics. As an example of a weak thermal lens, it is usually possible to design a laser resonator using Nd:YLF in a way that results in a thermal lens of focussing power that is weaker than that of other intracavity optics.
It is important to note that a diode-pump-induced thermal lens is not a perfect lens, but is rather an aberrated lens. In a typical high power diode-pumped laser design, a strong thermal lens is inherently an aberrated thermal lens. It is thought that the aberrations in the strong pump induced thermal lenses limit the efficiency of high power diode bar pumped lasers. See for example, S. C. Tidwell, J. F. Seamans, M. S. Bowers, A. K. Cousins, IEEE J. Quantum Electron. 24,997 (1992). This is because the thermally-induced aberrations add significant diffractive loss to resonators when conventional mode-matching techniques are employed. An aberrated thermal lens as one where the optical path differences a function of radius cannot be adequately fit by a simple parabola. A hypothetical perfect thermal lens would have an optical path difference as a function of radius that could be fit by a perfect parabola. For a typical aberrated thermal lens, the optical path difference as a function of radius is most nearly parabolic near its center, but deviates strongly from a parabola in its wings, as heat flows out of the pumped center into the surrounding crystal. See for example J. Frauchiger, P. Albers, H. P. Weber, IEEE J. Quantum Electron. 24, 1046 (1992).
It has been reported that the efficiency of a laser system with aberrated thermal lensing is reduced with respect to a laser system without aberrated thermal lensing because the thermal aberration acts as a pump-power dependent loss in the laser resonator. In order to make a relative comparison to a high power, high efficiency laser, this particular type of diode-pumped laser is used as a benchmark. The reference laser is a diode-bar-pumped Nd:YLF laser, as reported by S. B. Hutchinson, T. Baer, K. Cox, P. Gooding, D. Head, J. Hobbs, M. Keirstead, and G. Kintz, "Advances of 3-10 Watt Average Power Diode Pumped Lasers" in Diode Pumping of Average Power Solid State Lasers, G. F. Albrecht, R. J. Beach, S. P. Velsko, Editors, Proc. SPIE 1865, 61-72. The authors report a diode-bar-pumped Nd:YLF laser that was designed in a way that the thermal lens of the Nd:YLF could be considered weak, therefore presented only weak thermal aberrations. This laser provided 6 W of polarized output power (P0) in a TEM00 mode of M2&lt;1.1 for 17 W of diode-bar pump power incident (Pi) upon the Nd:YLF gain media. The optical efficiency (po/Pi) of this laser is .about.35%, while the optical slope efficiency (dP0/dPi) is .about.40%. This laser is a highly efficient, high power laser and can be considered a benchmark as a high power, highly efficient diode-bar-pumped laser that operates in a nearly diffraction-limited TEM00 mode. Any diode-bar-pumped laser with comparable power and optical efficiency in a nearly diffraction-limited TEM00 mode can therefore be called a highly efficient, high power, diode-bar-pumped laser.
High power diode-bar-pumped lasers that have been built using crystals that present strong thermal lenses have been reported to be less efficient than this benchmark. For example, overall optical efficiencies (Po/Pi) of only about 16% have been reported for end-pumped Nd:YAG operating in the TEM00 mode at the 6 W output level. The reported multimode efficiency achieved with Nd:YAG is higher, but multimode beams are not useful for many applications. See for example S. C. Tidwell, J. F. Seamans, M. S. Bowers, A. K. Cousins, IEEE J. Quantum Electron. 24, 997 (1992). A 26% optical efficiency (Po/Pi) was reported for an Nd:YAG laser at the 60 W level, but the TEM00 laser beam quality was worse than our benchmark at M2&lt;1.3, the beam was unpolarized, and the laser used an aspheric optic for aberration compensation that worked over only a narrow range of pump power. See for example S. C. Tidwell and J. F. Seamans, "60-W near TEM00, cw diode-end-pumped Nd:YAG laser", in Diode Pumping of Average Power Solid State Lasers, G. F. Albrecht, R. J. Beach, S. P. Velsko, Editors, Proc. SPIE 1865, 85-92. One report also indicated a 36% optical efficiency (Po/Pi) for a TEM00 Nd:YAG laser at the 7.6 W output level, pumped by 38 individual, polarization combined, fiber coupled diodes that provided an incident power of 21.1 W. A serious drawback of this system is that diode bars were not used, hence the tremendous complexity, cost, and low wallplug efficiency (Po divided by electrical input power) of 38 individual polarization combined fiber-coupled diodes. This was reported by Y. Kaneda, M. Oka, H. Masuda, and S. Kubota, "7.6 W of cw radiation in a TEM00 mode from a laser-diode-end-pumped Nd:YAG lasers", Opt. Lett. 17, 1003 (1992).
In spite of all of these difficulties, it would be very useful to develop a diode-bar-pumped laser that can make use of strong thermal lens materials and still operate at high power with high efficiency in a nearly diffraction-limited TEM00 mode. This is because some of these strong thermal lens materials have other desirable properties that make them desirable for certain applications. With respect to Nd:YLF, Nd:YV04 exhibits high gain and a short upper state lifetime. Nd:YAG has an intermediate gain and an intermediate upper state lifetime. These properties provide important adjustable parameters when designing a Q-switched laser with high pulse energy or high repetition rate, or a CW laser that is insensitive to optical feedback. Additionally, certain properties of Nd:YV04 make it attractive for diode pumping; its absorption coefficient at the diode pump wavelength of .about.809 nm is extremely high, permitting efficient coupling of diode pump light into the Nd:YV04 crystal.
Strong thermal lens materials like Nd:YAG and Nd:YV04 have been used in certain lasers with pump powers greater than 2 W. However, strong, aberrated thermal lenses are generated in end-pumped configurations. This is primarily because the index of refraction in these materials is a strong function of temperature, and the deposition of heat by the pump beam induces large thermal gradients. It has been generally believed that strong thermal lensing is a hindrance in the design and construction of an efficient laser with high beam quality, and therefore, there has not been great success in the use of strong thermal lens materials in highly efficient, high power diode-bar-pumped lasers.
Aberrations in the pump-induced thermal lens can be cancelled or corrected with specially shaped aspheric lenses or aspheric crystal faces within the laser cavity. Ideally, if the pump-induced aberrations are perfectly corrected, a favorable ration R (greater than unity) of TEM00 mode diameter to pump beam diameter can be employed, and optical efficiencies can approach those of more conventional mode-matched lasers that do not have significant aberration. There are, however, some significant limitations to these types of non-dynamic compensation schemes. They do not work well over a range of pump powers since the magnitude of the thermal lens, and its aberrations, is a dynamic function of the pump and intracavity powers. These designs have limited appeal because they work over a very small range of diode pump powers.
A need exists for highly efficient lasers. A low efficiency laser requires more diode pump power to achieve a desired laser output power. Increasing the pump power from a particular diode bar source increases the temperature of the diode junction, and the lifetime of the diode source is degraded in a predictable but highly undesirable way. This is unacceptable for applications that require long life. A low efficiency laser may require the use of additional diode sources to achieve a particular laser output power. This may be unacceptable for applications that are sensitive to cost or complexity.
There is also a need for a highly efficient, high power laser design that uses strong thermal lens materials and provides high quality beams over a range of pump powers. Additionally, it would be desirable to provide a diode-pumped laser employing strong thermal lens materials that can effectively operate in a Q-switched mode, or that can operate in a CW mode that is insensitive to optical feedback.
Finally there is a need for lasers that are acceptable for applications that are sensitive to cost or complexity.
Laser crystals with high thermally induced phase aberrations, such as Nd:YAG and ND:YVO4, have been considered for higher power diode pumped laser, having 2 W or greater output power. However, the thermal lens magnitudes in Nd:YAG and Nd:YVO4 are very large in high power end-pumped geometries because the variation of the indices of refraction of these materials as a function of temperature are large. Accompanying the large focal powers of the thermal lenses are high aberrations. It is reported that these large aberrations limit the efficiency of high power systems that use highly aberrating materials. Thermal aberrations in Nd:YLF, a material with low aberrations, are about an order of magnitude less in similar end-pumped geometries because the dn/dt is much smaller than for Nd:YAG or Nd:YVO4. It has been generally believed that strong thermal lensing is a hindrance in the design and construction of an efficient laser with high beam quality, and therefore, the use of high aberration materials at higher pump powers has been limited.
Aberrations in the pump-induced thermal lens can be cancelled or corrected with specially shaped aspheric lenses as aspheric crystal faces in the laser cavity. Ideally, if the pump-induced aberrations are perfectly corrected, a favorable ratio, greater than unity, of TEMOO mode diameter to pump beam diameter can be employed, and optical slope efficiencies can approach those of more conventional mode-matched lasers that do not have significant aberration. There are, however, some significant limitations to these types of non-dynamic compensations schemes. They do not work well over a range of pump powers since the magnitude of the thermal lens, and its aberrations, is a dynamic function of the pump and intracavity powers. These designs have limited appeal because they work over a very small range of pump powers.